This invention relates to a trans-capacitance amplifier used in electronic systems in which the bias of the input is realized by a circuit with an input current in the range of picoAmperes.
Electronic systems which process the voltage provided by a sensing device approximately modeled by a capacitor (such as a MEMS gyroscope or a MEMS microphone) use a continuous time, trans-capacitance amplifier using an operational amplifier. The operational amplifier feedback path comprises a capacitor. The sensing device provides a very low voltage, so it is critical that the current leakage due to the operational amplifier input biasing circuitry is minimized. Therefore, the biasing circuitry circuitry requires a very low input current (<10 pA).
A conventional technique uses a MOS transistor connected between the output and the input of the operational amplifier, and biased at a very low DC current. One drawback of this technique is that the current through the MOS transistor is modulated by the voltage swing at the output of the operational amplifier. Another drawback is that large operational amplifier voltage swings drive the MOS transistor in a non-linear regime, therefore limiting the linear regime of the entire trans-capacitance amplifier. A refinement of this conventional technique uses a resistive divider at the output, with the MOS transistor connected between the divided voltage and the input of the operational amplifier. The effect of the modulation of the MOS transistor by the output voltage is reduced, however the noise of the amplifier is increased, the equivalent output-referred voltage offset is increased, and the operational amplifier must source the DC current flowing through the resistive divider.
Another conventional technique uses a trans-conductance amplifier comprising a differential, common-source MOS pair having an active load, the said MOS pair being biased in the sub-threshold region; wherein the said trans-conductance amplifier has its output connected to its inverting input, and the said trans-conductance amplifier has its non-inverting input connected to the said operational amplifier output. This technique also has the drawback of the modulation of its output current by the operational amplifier output signal and the drawback of a limited output voltage swing. A refinement of this conventional technique degenerates the said MOS pair by adding diodes in series with their sources, hence reducing the trans-conductance of the said differential pair and further reducing the said modulation of its output current. However, even using this technique, an input current in the range of hundreds of picoAmperes together with a large linear range of the operational amplifier output voltage cannot be obtained (the said linear range is limited at most to a couple hundreds mV).